Processing image representative data often requires or benefits from data reduction in the form of data compression coding. Data compression is beneficial, for example, in the context of digital video recording.
A conventional data compression method comprises transforming blocks of image pixels, e.g., 8.times.8 pixel blocks by means of a DCT (discrete cosine transformation). Such a process is described, for example, in an article "Scene Adaptive Coder", W. H. Chen and W. K. Pratt, IEEE Transactions on Communications, Vol. COM32, No. 3, March, 1984. Data reduction is made possible by the fact that the pixels within such blocks are correlated with one another, and the important transformation coefficients are therefore concentrated in a few locations within each transformed block. Thus only a few coefficients per block need to be coded rather than 64 (8.times.8 pixels). Conventional television standards such as PAL, SECAM, and NTSC form an image frame based upon the information contained in a pair of successive image fields. If two successive fields are combined, and the pixel blocks to be transformed are taken from these fields, the pixels in these blocks, which come from the respective fields, may exhibit little spatial correlation with those from the other field in the case of dynamic (e.g., moving) image content. As a result, transformation coding can be ineffective.
Transformation coding adapted to static and dynamic image content has been proposed. For example in German DE 3642664 it has been proposed to switch between an 8.times.8 DCT and two corresponding 4.times.8 DCTs per block. An 8.times.8 DCT is performed for the respective block with static image content, whereas two 4.times.8 DCT transformations are performed for the respective block with dynamic image content, separately for the pixels of the respective field. However, this means that apparatus using this method must be able to perform two types of transformation, the implementation of which is relatively complex.